The invention relates to a colour television display tube comprising an envelope having a substantially rectangular display window and a substantially rectangular colour selection electrode which comprises a large number of apertures and is suspended in the corners of the display window with the aid of suspension means comprising flat resilient elements.
Such a colour television display tube is disclosed in U.S. Pat. No. 3,548,235. In this patent a construction is described for the suspension of a colour selection electrode, in particular a shadow mask, in which a rigid supporting frame connected to the shadow mask is not used. The shadow mask is formed by a substantially rectangular, comparatively flexible mask sheet manufactured from a thin metal sheet and provided with a large number of apertures. A mask ring of substantially the same thickness as the mask sheet is connected to the edge. The mask ring gives the shadow mask a certain rigidity in the direction of the rectangular sides. However, the shadow mask can be comparatively easily twisted about the diagonals and consequently has four hinge points at the corners. The shadow mask is suspended in the four corners of the display window with the aid of suspension means of which the part connected to the display window is formed by clamping springs which are clamped in chamber-like recesses in the corners of the display window. Flat resilient elements in the form of strip-shaped springs are connected to the clamping springs and permit expansion of the shadow mask when the temperature increases. The strip-shaped springs are connected to the shadow mask at the end remote from the clamping springs.
In such a suspension of the shadow mask, the desired distance between the shadow mask and the display window is established by four suspensions points in the corners of the shadow mask because with only three the shadow mask could be twisted about a diagonal. The position of the shadow mask relative to the display window in directions parallel to the display window is determined by three of the four suspension points. The position of the fourth suspension point is thus fixed by the other three points. The fourth suspension point should consequently assume an equilibrium position relative to the other three points. The position of the fourth suspension point is also determined by the position of the clamping spring in the chamber-like recess. As a result of this in the known tube moments are exerted on the shadow mask by the suspension means causing the shadow mask to be deformed. Moreover, in such a tube the forces which occur in the case of shocks or vibrations of the display tube cause movements of the shadow mask, which causes fading of the displayed picture.
U.S. Pat. No. 3,999,098 discloses a suspension of a shadow mask in the corners of the display window in which the parts of the suspension means connected to the shadow mask comprise very rigid leaf springs extending substantially parallel to the tube axis. Three of the four leaf springs include an aperture. The fourth leaf spring include a tapering mandril. The part of the suspension means connected to the display window is formed by four metal supports sealed in the corners of the display window and extending substantially perpendicularly to the diagonals of the display window. Three of the four supports include a tapering mandril. The fourth supporting member includes a substantially rectangular slot. The leaf springs having the apertures fall over the mandrils of the supports and the mandril of the fourth leaf spring falls into the rectangular slot of the fourth support. The fourth suspension point together with the three other suspension points determines the desired distance between the shadow mask and the display window. The position of the mask sheet in the plane thereof is determined by the three identical suspension points in the corners of the shadow mask. The mandril of the leaf spring of the fourth suspension point, in the rectangular slot of the fourth supporting member, seeks an equilibrium position for the mask sheet.
In such a suspension system the natural rigidity of the shadow mask is relied on to counteract the forces extended thereon by the leaf springs. Moreover, the position of the shadow mask is not unambiguously determined since the position of the fourth suspension point is not unambiguously determined. Thus, poor assembly reproducibility is obtained when the shadow mask is repeatedly assembled and disassembled in the tube. This repeated assembly and disassembly of the shadow mask is necessary when photographically producing the display screen. Assembly reproducibility is to be understood to mean the extent to which the shadow mask resumes the same position after each disassembly and assembly.